To use Autofac in your project, all you have to do is install Autofac from NuGet Package Manager. Open the solution that want to use Autofac in, then select Manager NuGet Packages for Solution... by going to:

In the NuGet-Solution tab, type in "Autofac" in the search box. Make sure you are in the "Browse" section. Install the first option as shown in the image below (take note of the marked regions in the image):

Installing through NuGet will automatically add Autofac in the References of the projects which were selected during installation.